An example gas reaction process is widely used for the production of ammonia. For more than a century, the conversion of nitrogen and hydrogen gases into ammonia has been known and commercialized, but the conventional process involves heating the “feedstock” gases and is terribly energy-inefficient. In fact, approximately 2% of the total electrical production worldwide is devoted to the production of ammonia for the agricultural industry.
Plasma systems for generating specific chemical products have been investigated extensively over the past two decades, but the only commercially successful example of such reactors are those generating ozone for the disinfection of drinking water and wastewater treatment. Photochemical systems, in which light dissociates a molecule for the purpose of forming a new molecule, have proven to be of modest industrial value, with the exception of the photochemical production of Vitamin D for incorporation into milk and other products.
Plasma-chemical reactors and processes seek to use plasma to initiate desirable chemical reactions. Plasma can be used to promote chemical reactions in liquids and gases, and on the surfaces of solids. Present commercial plasma systems are used for printing, for treating water and for sterilizing surfaces, for example. An impediment to the wider adoption of commercial plasma-chemical reactors is the scale and expense of conventional atmospheric pressure plasma technology. The cost, size, weight, and high voltages characteristic of typical plasma-chemical reactors limit the commercial potential of conventional plasma reactor technology. Ozone treatment is a particularly attractive application of plasma-chemical technology but the cost, size, and weight of most existing systems restrict their value for many commercial uses.
Ozone can be produced when oxygen (O2) molecules are dissociated by an energy source into oxygen atoms. Collisions of free oxygen atoms with oxygen molecules produce ozone (O3) which is typically generated at the point of treatment because the lifetime of O3 in air at atmospheric pressure is on the order of minutes. Ozone is the strongest oxidant and disinfectant available commercially. Mechanisms of disinfection using ozone include direct oxidation/destruction of bacterial cell walls, reactions with radical by-products of ozone decomposition, and damage to the constituents of nucleic acids. Presently available dielectric barrier discharge (DBD) systems for industrial scale production of ozone for municipal water treatment, for example, are large (up to 10-15 ft. in length) and have demanding power requirements (150-200 kVA). Furthermore, the conversion of feedstock gases into O3 is often inefficient, thereby raising electrical power consumption and cost. Existing commercial processes for producing O3 in large volume typically convert 15%-18% of the oxygen (O2) feedstock gas into O3. This low efficiency for the conversion of feedstock gas to ozone is a result of the fact that ozone is produced only within, or in the vicinity of, the streamers produced in air or oxygen by large volume DBD systems. Maintenance of such systems is also problematic owing to a large number of ceramic parts and fouling of device components by nitric acid. Existing dielectric barrier discharge technology is also sensitive to the level of organic impurities in the oxygen feedstock gas.
There are additional drawbacks to existing commercial plasma-chemical devices and systems. Dielectric barrier discharge structures, commonly used in present day commercial plasma systems operating at atmospheric pressure, are uncomplicated devices which apply high voltages to electrodes separated by a dielectric (often, glass or quartz) and the gas or vapor in which plasma is to be produced. Typical macroscopic reactors rely upon microdischarge streamers that are nominally 100 μm in diameter and statistically distributed in space and time. Efficiencies for the conversion of gas feedstock reactant(s) into the desired product are low which, for ozone generation, requires large volumes of oxygen (or air) flows to generate reasonable amounts of O3. Moisture and organic contaminants in the feedstock gas are another problem with conventional ozone generating systems because the system can be fouled and rendered less efficient, or disabled, as a result of nitric acid build up on the reactor wall or on vacuum fittings. Similar difficulties are faced when attempting to process other gases such as carbon dioxide or water vapor in atmospheric pressure DBD-produced plasmas.
A portable ozone generator is described in U.S. Pat. No. 7,157,721 (“'721 patent”). In the '721 device, both sides of a glass or ceramic plate are coated with conductive materials to form electrodes having different areas. Such a device produces a corona discharge in the region lying outside the smaller of the two electrodes. An ozone device based upon this corona discharge mixes ozone with water in flow channels that are formed in plastic. No microchannels exist in the ozone-producing reactor. Another manufacturer provides a modular approach to ozone generation that is based upon corona discharge cells. However, because the corona discharge reactors are not flat and the plasma is not confined to microscopic channels, these reactors are not readily or easily combined and, in particular, are not amenable to being stacked. Furthermore, the voltages required of corona discharge systems are high (multi-kV) and conversion efficiencies (oxygen or air →ozone) are low.
The present inventors and colleagues have developed microplasma devices in various materials, including microcavity plasma devices and microchannel plasma devices. Microplasma devices are disclosed, for example, in the following patents, incorporated by reference herein. U.S. Pat. No. 8,968,668, entitled Arrays of metal and metal oxide microplasma devices with defect free oxide; U.S. Pat. No. 8,890,409, entitled Microcavity and microchannel plasma device arrays in a single, unitary sheet; U.S. Pat. No. 8,890,409, entitled Microcavity and microchannel plasma device arrays in a single, unitary sheet; U.S. Pat. No. 8,870,618, entitled, Encapsulated metal microtip microplasma device and array fabrication methods; U.S. Pat. No. 8,864,542, entitled Polymer microcavity and microchannel device and array fabrication method; U.S. Pat. No. 8,547,004, entitled Encapsulated metal microtip microplasma devices, arrays and fabrication methods; U.S. Pat. No. 8,497,631, entitled Polymer microcavity and microchannel devices and fabrication method; U.S. Pat. No. 8,492,744, entitled Semiconducting microcavity and microchannel plasma devices; U.S. Pat. No. 8,442,091, entitled Microchannel laser having microplasma gain media; U.S. Pat. No. 7,573,202, entitled Metal/dielectric multilayer microdischarge devices and arrays; U.S. Pat. No. 7,482,750, entitled Plasma extraction microcavity plasma device and method.
Another example device developed by several of the present inventors and colleagues produces low temperature plasma in microchannels. Specifically, Park et al. U.S. Pat. No. 8,442,091, incorporated by reference herein, discloses microchannel lasers having a microplasma gain medium. In that patent, microplasma acts as a gain medium with the electrodes sustaining the plasma in the microchannel Reflectors can be used in conjunction with the microchannel for obtaining optical feedback and lasing in the microplasma medium in devices of the invention for a wide range of atomic and molecular species. Several atomic and molecular gain media will produce sufficiently high gain coefficients that reflectors (mirrors) are not necessary. FIG. 4 of that patent also discloses a microchemical reactor that is suitable for air purification and ozone production because of the channel lengths and large power loadings (watts deposited per unit volume) of the plasma that are available. However, fabrication costs associated with channels of extended length present an obstacle to commercialization of this technology for applications that would benefit from ozone production.
Some of the present inventors and colleagues have developed other microplasma devices that produce high quality plasmas (i.e., uniform glows) in microchannels. For example, linear arrays of 25-200 μm wide channels have been fabricated in glass by replica molding and micropowder blasting and have been demonstrated to be capable of generating low temperature, nonequilibrium microplasmas. See, Sung, Hwang, Park and Eden, “Interchannel optical coupling within arrays of linear microplasmas generated in 25-200 μm wide glass channels,” Appl. Phys. Lett. 97, 231502 (2010). Parallel microchannels have also been fabricated in nanostructured alumina (Al2O3) via a nanopowder blasting process, and shown to provide the capability for routing, and controlling the flow of, packets of low temperature, nonequilibrium plasma. See, Cho, Park and Eden, “Propagation and decay of low temperature plasma packets in arrays of dielectric microchannels,” Appl. Phys. Lett. 101, 253508 (2012). Further development and research on these and additional microchannel structures by some of the present inventors and colleagues have resulted in the realization of ozone microreactors capable of generating ozone and fragmenting other gas molecules. See, [0062]-[0066] of commonly owned Eden et al., US Published Patent Application 2013/0071297, published Mar. 21, 2013. The ozone microreactor in the '297 application included 12 microchannels that supported a flow rate of 0.5 standard liters per minute (slm) and ozone generation efficiencies exceeding 150 g/kWh.
A modular approach is provided in Eden et al. U.S. Pat. No. 9,390,894, which is incorporated by reference herein. That patent discloses modular microchannel microplasma reactors, reactor modules and modular reactor systems that include pluralities of the modular microchannel reactors and reactor modules. The reactors, reactor modules, and modular systems are readily combined and scaled into large systems.
A photon emitting microcavity lamp has previously been patented by the inventor and colleagues U.S. Pat. No. 6,194,833, entitled Microdischarge lamp and array, which is incorporated herein. With an appropriate medium, the lamp can emit deep UV photons.
Prior UV/VUV lamps have been produced commercially, but are generally expensive, bulky and require a cylindrical geometry. Such lamps are available from Hamamatsu, Heraeus, and other manufacturers.
A series of high power and efficient ultraviolet/vacuum ultraviolet (UV/VUV) lamps was recently demonstrated by Eden Park Illumination of Champaign, Ill. One product is referred to as the Vacuum UltraViolet Lighting System, and provides mercury-free 172 nm (photon energy of 7.2 eV) radiation from an example 4″×4″ (100 sq. cm) flat surface. More than 25 W of average power and greater than 600 W of peak power have been produced from such lamps. Park et al. describe the performance of the 172 nm lamp in the publication “25 W of average power at 172 nm in the vacuum ultraviolet from flat, efficient lamps driven by interlaced arrays of microcavity plasmas,” APL Photonics 2, 041302 (2017). The overall (“wallplug”) efficiency of these lamps is currently above 23%. These numbers are unprecedented in the deep UV and the VUV (wavelengths between 100 nm and 250 nm). Eden Park Illumination also provides flat microcavity VUV lamps at longer wavelengths, including some that can operate over a range of wavelengths, e.g., 220-260 nm, and others that operate at specific wavelengths, e.g., 185, 194, 207, 222, 226, and 308 nm.
Despite this accomplishment, however, the cost of photons in this spectral region remains high and reactors based solely on photochemistry do not appear to be attractive for many commercial processes at present. One reason for this assessment is that these lamps are available at present at only a few selected wavelengths that are not absorbed strongly by several prominent molecules of interest (e.g. carbon dioxide, methane, hydrogen, and nitrogen).